1. Field of the Invention
The present invention relates to a converting technique for magnifying or reducing the size of a picture of an interlace system by an arbitrary integer ratio.
2. Description of the Prior Art
A prior art technique for magnifying or reducing an original picture of an interlace system by an arbitrary integer ratio is achieved by manipulating, that is, by increasing or decreasing the number of picture elements with respect to the original picture and by rearranging the manipulated picture elements at the same interval as that of the original picture.
In the above prior art technique, in order to magnify the original picture two times, for example, an interpolation operation which doubles the number of picture elements is performed.
However, as shown in FIG. 1, if an original picture element is interpolated so as to double the number of the picture elements for every field, or to magnify the original picture, an interlaced picture cannot be formed. In FIG. 1, circles represent original picture elements and triangles represent interpolated picture elements. Further, the longitudinal axis represents the vertical direction of a picture and a horizontal axis (not shown) represents a time axis. F1 and F2 are two fields which may be interlaced with each other.
As a result, linear interpolation is utilized to provide an interlaced picture as shown in FIG. 2. In FIG. 2, circles having broken lines represent original picture elements which disappear after the interpolation; circles having solid lines represent interpolation picture elements whose positions correspond to those of the original picture elements after the interpolation; and triangles represent interpolation picture elements whose positions do not correspond to those of the original picture elements after the interpolation. However, in the linear interpolation shown in FIG. 2., a problem known as flickering occurs in the picture as hereinafter described.
In linear interpolation, the two picture elements which are the closest to an interpolation point are selected, and the picture element data of the two picture elements are divided by the ratio of the distances between two picture elements. As shown in FIGS. 3 and 4, the distance between the original picture elements P1 and P2 is D, and a picture element which is to be interpolated is indicated by .DELTA. and is located at a distance x from the original picture element P1. Assume that the picture element data of the original picture elements P1 and P2 are T and Q, respectively. As a result a value R of the interpolation picture element may be expressed by the following equation: ##EQU1##
The linear interpolation can be achieved by a filter having a characteristic of a linear impulse response.
In FIGS. 3 and 4, each value of the eight interpolation points *0 - *7 is obtained by the following equations: ##EQU2##
The interpolation of each of the interpolation points *0 - *7 is attained by a subfilter having a filtering coefficient which is provided by sampling an impulse response (interpolation function). The subfilter may include a well-known FIR type digital filter with two taps.
Thus, the value of each of the interpolation values *0 to *7 shown in FIGS. 3 and 4 is obtained using a subfilter having a coefficient corresponding to the above equations.
As is to be appreciated from the above-mentioned equations (2) to (9), if the impulse response is symmetrical, then the sets of the subfilters have combinations of the same coefficient sets. For example, if the impulse response is represented by either equations (3) and (9), (4) and (8) or (5) and (7), then the corresponding sets of subfilters would have the same characteristic even though the arrangement of the coefficients is reverse. More specifically, interpolation points *1 and *7, *2 and *6, and *3 and *5 are interpolated by respective subfilters having the same respective characteristic. Therefore, in the interpolation shown in FIG. 2, only the equations (4) and (8) in the first field F1 and the equations (2) and (6) in the second field F2 are utilized. As a result, since the sets of subfilters have different characteristics, flickering tends to occur.